IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Tuesday Sessions
       Session SS3-TuP

Paper SS3-TuP22
Hyperthermal Product Velocity Detected as a Signature of Electron Harpooning in Gas-Surface Reactions

Tuesday, October 30, 2001, 5:30 pm, Room 134/135

Session: Adsorption/Desorption Poster Session
Presenter: G.C. Poon, University of California, San Diego
Authors: G.C. Poon, University of California, San Diego
J.-C. Gumy, University of California, San Diego
K.A. Pettus, University of California, San Diego
A.C. Kummel, University of California, San Diego
Correspondent: Click to Email
Multiphoton Ionization (MPI) and Time-of-Flight Mass Spectroscopy (TOF-MS) have been used to demonstrate that the reaction of ICl on the low work function Al(111) surface proceeds by a nonadiabatic electron harpooning process. In the nonadiabatic model of this reaction, as ICl approaches the Al(111) surface, an electron harpoons from the surface, suddenly converting ICl to ICl@super -@. This places the molecule high on the repulsive portion of the ICl@super -@ potential curve leading to rapid dissociation of ICl@super -@ into I@super -@ and Cl fragments. Following this remote dissociation of ICl above the surface, I@super -@ proceeds towards the surface and sticks while Cl is ejected into the gas phase. This is referred to as chemically selective abstraction and is consistent with the observed excess iodine on the surface by Auger Electron Spectroscopy and the detection of ejected Cl atoms. The experimentally observed signature of this harpooning process is the hyperthermal translational energy of the ejected fragment. A harpoon process should give fast Cl whose energy is determined by the vertical transition between ICl and ICl@super -@ and be independent of incident translational energy. Conversely, a conventional adiabatic abstraction reaction should provide only a small translational energy for the ejected Cl and should increase with increased incident translational energy. Hyperthermal Cl (0.36 eV ± 0.14 eV) ejected from the surface has been observed using MPI/TOF-MS, which is evidence for the aforementioned nonadiabatic process. Furthermore, DFT calculations of the vertical transition energy from ICl to ICl@super -@ are in good agreement with experiment. Comparison of the translational energy of ejected Cl from the abstractive chemisorption of Cl@sub 2@ and ICl will also be presented. Cl@sub 2@ should lead to even faster Cl ejected from the surface than for ICl, since Cl@sub 2@@super -@ is formed even higher on the repulsive portion of the potential curve.